1. Field of the Invention:
This invention relates to a magnetic resonance imaging (MRI) system for obtaining magnetic resonance (MR) images of a selected slice of an object which may involve, a living subject. More specifically, this invention relates to an MRI system capable of reducing the degradation in the MR images which is caused by image artifacts or false images resulting from variations in MR information obtained from regions of the object, such as blood vessels whose MR information periodically varies due to the varying flow rate of blood.
b 2. Description of the related art including
information disclosed under .sctn..sctn.1.97-1.99
In MRI systems, a subject (patient) on an examination table or couch is placed in a static homogeneous magnetic field, and a high frequency magnetic field, usually RF (radio frequency) magnetic field is applied to the subject in a direction orthogonal to the static magnetic field. As a result, a magnetic resonance phenomenon is induced in nuclear spins in the subject. After the removal of the RF magnetic field, a magnetic resonance (MR) signal generated by the resonated nuclear spins is detected and collected to form an MR image. For the excitation of the magnetic resonance and the collection of the MR signal, in general, some gradient magnetic fields are applied to a predetermined portion of the subject.
As one of typical imaging methods for the MRI systems the two-dimensional Fourier transform (2DFT) method is well known, which uses a technique called phase encoding method in order to obtain two-dimensional position information in a selected slice of a subject.
Where this phase encoding method is used, a pulsed RF magnetic field, i.e. an RF pulse, and plural pulsed gradient magnetic fields are used. The gradient magnetic fields involve a slicing gradient magnetic field adapted for selecting a slice of the subject to obtain an MR image, and also a phase encoding gradient magnetic field and a readout gradient magnetic field, both adapted for adding two-dimensional position information to a detected MR signal.
Reference is now made to FIGS. 1A through 1E to describe a pulse sequence of the RF pulse and gradient magnetic fields in the phase encoding method.
A 90.degree. RF pulse and a 180.degree. RF pulse in FIG. 1A, a slicing gradient magnetic field Gz in FIG. 1B, a phase encoding gradient magnetic field Gy in FIG. 1C, and a readout gradient magnetic field Gx in FIG. 1D are generated during a repetition time TR at respective timings shown in FIGS. 1A through 1D. This pulse sequence is repeated. The encoding gradient magnetic field Gy has its intensity (the magnitude of gradient) changed by a predetermined amount each time the pulse sequence is repeated. In this way, the MR signal can be gathered which include two-dimensional position information about the selected slice of the subject. The MR signal include an FID (free induction decay) signal and a spin-echo signal. However, the widely used spin-echo method detects and collects the spin-echo signal only.
The collected MR signal is then subjected to the two-dimensional Fourier transform, i.e. Fourier transforms in the x and y directions to form or reconstruct an MR image for the selected slice of the subject.
According to the existing systems, however, where a slice of the subject to be examined, or a reconstructed MR image thereof includes such an MR-information varying portion as an image of an aorta whose MR information varies owing to varying flow rate of blood, image artifacts would appear in the finally obtained MR image which are due to the variation in the MR information. This adversely affects the diagnosis of the subject under examination based on the MR image.
To collect the MR information within a sufficiently short time as compared to the rate of MR-information variation may prevent the image artifacts from being generated. It is impossible, however, to collect enough MR signals to reconstruct an MR image within such a short time. An existing system uses a technique for collecting an MR signal within a very short time, in synchronism with a cycle of variation of an MR-information varying portion and in a specified phase condition of the MR-information variation, in which case, however, it requires a long period of time to collect enough MR signals to reconstruct an MR image.